Resistive ribbon thermal transfer printing is well known in the art as a type of nonimpact printing. Printing is effected by the flow of a melted material (ink) from a transfer medium to a recording medium, such as paper. A ribbon is used having a resistive layer, a metal (Al) current return layer, and an ink layer. In some ribbons, an ink release layer is located between the metal layer and the ink layer in order to facilitate the transfer of ink from the ribbon to the paper. In operation, electrical currents flow from printing electrodes into the resistive layer to a thin Al current return layer. This flow of current causes localized heating which melts the ink, allowing it to transfer to the paper which contacts the ink layer. High quality printing of the type used for computer terminal applications and typewriters is thereby possible.
To accomplish high quality printing, many factors have to be present in the performance of these ribbons. One factor is the response of the resistive layer to the applied current, with respect to the current required for adequate heating and with respect to the avoidance and degradation of the ribbon or printhead from the effects of heating and current flow. To accomplish the required printing, the resistivity and other properties of the resistive layer are carefully controlled during fabrication.
In general, a contact resistance exists between the sliding printing electrodes and the resistive ribbon, due to imperfect contact between these members. Some proportion of the power supplied to the ribbon is dissipated by this contact resistance, resulting in heating of the print head and causing undesired wear and other consequences as well as inefficient use of energy. Further, there tends to be abrasive wear of the hot electrodes as they slide across the ribbon surface. It is therefore advantageous to have low contact resistance so that heating of the surface of the ribbon and the head is minimal, a factor which is especially important for high speed printing in which higher print currents are generally employed.
Contact resistance may be reduced by coating the top surface of the ribbon with a highly conductive material, as illustrated in the following references: U.S. Pat. Nos. 4,309,117; 4,453,839; 4,477,198; and IBM Technical Disclosure Bulletins, appearing in Vol. 25, No. 7A, December 1982, at pages 3193 and 3194. In the first of these listed patents a two-ply resistive layer has a top-ply consisting of a low resistance material and a bottom-ply of high resistance material. The other cited patents and the Technical Disclosure Bulletins generally describe various embodiments using graphite for reduction of contact resistance. In U.S. Pat. No. 4,453,839 the resistive layer includes a light dusting of graphite while in U.S. Pat. No. 4,477,198 the resistive ribbon has a more extensive coating of graphite powder on one side of the resistive layer. The graphite powder provides lubrication and enhanced electrical current-flow parameters.
The Technical Disclosure Bulletins describe resistive ribbons for thermal transfer printing which utilizes either a graphite-resin layer or an embedded graphite layer to improve the current-flow characteristics of the ribbon. In particular, the printing current requirements are reduced and their build-up of free graphite on the printhead is avoided, as is the transfer of graphite to the ink layer. The graphite also appears to reduce frictional wear on the printhead.
Although the technology has recognized that contact resistance may be reduced by coating the top surface of the ribbon with a highly conductive material, such solutions may be unsatisfactory due to the spreading of current from the printing electrodes. Thus, while a more conductive layer is required to reduce contact resistance, the resistivity of this layer may be so low that printing current spreads in the layer and thereby causes a loss in print resolution. In order to lower contact resistance and also to reduce spreading of the current, the coating layer must have a resistivity lower than that of the resistive layer in the ribbon. In addition, the sheet resistivity of the coating layer must be much higher than the sheet resistivity of the resistive layer in the ribbon. This means that the thickness of the coating material must be below a certain limit, which is determined in accordance with the materials used for both the coating layer and the resistive layer in the ribbon.
The considerations described hereinabove with respect to resistivity and sheet resistivity have been recognized herein as being critical to the provision of a suitable layer for reducing contact resistance. The selection of a material to satisfy these properties and yet be readily fabricated as a thin layer having a sufficient degree of flexibility to be wound on a ribbonbearing reel is not readily apparent. Further, while the contact layer should lower the power required for printing and reduce heating of the printhead, it must be a material which is extremely stable so as to have long shelf life, and in addition must be stable during the actual printing operation. This means that it must not be readily corrodible and that it won't be damaged, as by erosion, during printing. Still further, the contact resistance-reducing layer must adhere well to the resistive layer and be sufficiently thin that the total printing capacity of the ribbon is not substantially reduced.
As noted, both sheet resistivity and bulk resistivity must be within certain ranges in order to provide effective contact resistance layers without impairing the printing operation. With materials such as graphite, it is very difficult to control the thickness and uniformity of the coating. Additionally, with metallic materials such as Cu, the conductivity is so high that the materials must be produced as extremely thin coatings. This in turn provides conductive films which are easily eroded during printing and which are subject to corrosion. Thus, with the materials used as contact resistance coatings in the prior art, there is inadequate control of the reproducibility of resistivity and thickness. This coupled with the often difficult fabrication processes and the inadequate electrical properties of the entire ribbon including the coating, has limited use of these prior coatings.
Accordingly, it is an object of this invention to provide an improved coating for reducing the contact resistance of a resistive ribbon used for thermal transfer printing.
It is another object of this invention to provide an improved resistive printing ribbon having a coating thereon for reducing contact resistance, where the coating can be easily fabricated with an appropriate resistivity and thickness.
It is another object of this invention to provide an improved resistive printing ribbon having a coating thereon for reduction of contact resistance, the coating being stable during ribbon storage and during actual printing operations, said coating adhering well to the resistive layer in the ribbon.
It is another object of the present invention to provide an improved coating for reducing contact resistance in a resistive printing ribbon, where the coating can be sufficiently thin that the total printing capacity of the ribbon is not substantially reduced.
It is another object of the present invention to provide an improved resistive ribbon for thermal transfer printing having a coating thereon which reduces contact resistance, where the coating can be made to have a very smooth surface.